“Why should earth be the only planet supporting human life?” asked the physicist in 1920.

The release this month of a digital edition of Albert Einstein’s papers had me hunting through the great man’s works for any mention of space travel. True, Einstein died two years before the launch of Sputnik 1 in 1957, but people had been theorizing about rocket flight for decades by then, including some, like Hermann Oberth, in the physicist’s native Germany.

Unfortunately, this first digital release only covers Einstein’s life up until 1923, around the time Oberth started writing about spaceflight. So not a single hit turned up for the search terms “rocket” or “space travel.” Guess we’ll have to wait for the later volumes.

There was, however, an intriguing item from January 1920, a reference to an article in the London Daily Mail, whose correspondent had asked the soon-to-be Nobel laureate his opinions about extraterrestrial life. Radio pioneer Guglielmo Marconi had recently told the same paper about mysterious signals he speculated may have come from Mars. What did Einstein think?

“There is every reason to believe that Mars and other planets are inhabited,” answered the professor. “Why should the earth be the only planet supporting human life? It is not singular in any other respect. But if intelligent creatures do exist, as we may assume they do elsewhere in the universe, I should not expect them to try to communicate with the earth by wireless [radio]. Light rays, the direction of which can be controlled much more easily, would more probably be the first method attempted.”

Einstein’s dismissal of what we now know as radio SETI (Search for Extraterrestrial Intelligence) may sound surprising, considering that’s exactly the region of the spectrum where most searches have been conducted to date. But in 1920, Einstein lacked a key bit of information. The reason radio searches are favored for modern SETI is that long radio waves more easily penetrate the pervasive dust in interstellar space that blocks shorter-wavelength light from reaching us. In 1920, astronomers didn’t yet understand the nature of interstellar dust.

The father of relativity theory may in fact have been right. Optical SETI searches by teams at Harvard and other places are enjoying a kind of revival, even though they have the disadvantage of searching only for light beams that would have to be aimed deliberately in our direction, as opposed to radio signals that spread out like ripples in a pond.

By the way, searching for the word “Mars” in Einstein’s digital papers also turned up this perhaps uncharacteristically misanthropic line from a letter sent in February 1917: “It is a pity that we do not live on Mars and just observe man’s nasty antics by telescope.”